In condemning the killing of Steven Sotloff, today President Obama said the U.S. will not be intimidated. Our reach is long, the president warned, and justice will be served. There are those who argue that any military strikes against the Islamic State should be authorized explicitly by Congress.

Democratic Senator Tim Kaine of Virginia has been outspoken in favor of that. We reached him today in Morocco where he's leading a congressional delegation. And I asked him if the beheadings of the two American journalists have altered his thinking.

SENATOR TIM KAINE: They don't alter my thinking, but I do think they increase the likelihood that Congress will support the president in a request for military authorization. It was after the recess began that the president first ordered airstrikes, and then we've had both examples of these horrible beheadings of American journalists. I think the president will find Congress very willing to work with him when we return on Monday to craft a crisp definition of what that mission against ISIL should be.

CORNISH: There doesn't seem to have been an appetite among your colleagues, though, for a debate or a vote on this as we head into midterm elections. A number of your fellow Democrats say why put members of Congress in the position of voting on a measure that would be unpopular among their base - could hurt them on election day?

KAINE: Well, the reason we should do it, Melissa, is because we volunteered for the job, and this is the most sacred part of the job, and it is a responsibility that I will never, never cede to the president. The framers of the Constitution indicated that war should only be declared by Congress.

If we're going to ask American servicemen and women to risk their lives, we shouldn't do it unless there is a political consensus that the mission is worthwhile. But in this instance, the threat is serious enough. And I think the horrific events of the last few weeks will guarantee significant congressional cooperation.

CORNISH: But couldn't the president make a clear case - especially now, given these executions - that the objective is protecting Americans abroad and that is clearly within his authority under the Constitution - under Article Two of the Constitution?

KAINE: The framers of the Constitution had a very clear view that - your correct, Melissa, that the president could act under some circumstances under Article Two. But even in that instance, the framers were very clear that the president should loop back to get congressional authorization.

So the attack of an American embassy or American property or American servicemen and women - the president could defend the nation prior to, but not without congressional approval. I think it would be a mistake for the president not to come to Congress on this.

CORNISH: We heard President Obama say today, our objective is clear and that is degrade and destroy ISIL so that it's no longer a threat. Do you share the objective that ISIL should be destroyed by the United States?

KAINE: Well, I think the operative is the last part - so that it's no longer a threat. There's never an instance where military force alone does the trick. If we want to degrade ISIL - for example, the vigorous nature of its recruiting - there's all kinds of things that we can do. But I do believe that military action is a component of dealing with a group that poses as significant and violent a threat as ISIS does.

CORNISH: So you would support airstrikes in both Iraq and Syria, were it to come to that?

KAINE: I am prepared to if I feel like the President lays out a crisp definition of the mission. I want to see what the White House puts on the table. The statements last week that suggested we don't yet have a strategy were not confidence-building. Hopefully they are able to define with some reasonable precision what they would hope to accomplish with airstrikes. Doing one airstrike on Syria - well, this will protect a dam. And then we'll do one next week on this - we'll protect an ethnic group.

One-off decisions by the president without congressional discussion doesn't give the American people the education about, here's what's at stake. Here's why we need to act. The way you bring the American public into the equation is with that kind of deliberation and discussion in Congress.

CORNISH: If the threat posed by ISIS is the fast-moving train that the administration has portrayed, is there really time for the deliberation that you would want to see in Congress and the debate and the vote that would be taken?

KAINE: Yes, there is. If it's fast-moving, we can decide quickly. It can seem messy 'cause Congress is messy, but a president who brings a discussion like this to Congress and engages in a debate - it's more likely to work out better, but especially it puts it into an arena where we act only after we've reached a consensus.

CORNISH: We did hear the killer on the recent video that shows the execution of Steven Sotloff say, I'm back, Obama, because of your arrogant foreign policy and your insistence on continuing your bombings in Iraq. Those were the killer's words. Is it your view that those airstrikes have backfired?

KAINE: No. I think, generally - look, these are violent people who are doing violent things. I actually think some of the airstrikes seem to have had positive success in slowing the momentum of ISIL toward Baghdad - of retaking infrastructure such as the dam near Mosul. My assessment - and I'll want to dig in and ask these kinds of questions when the president hopefully comes to Congress - but my assessment is that the airstrike campaigns have achieved some humanitarian goals and have actually slowed ISIL's momentum.